Conversing by telephone in a noisy environment can be both difficult and annoying Interfering background noise may be transmitted with the voice signal to the person being called and fed back to the caller. Consequently, person being called may hear speech intermingled with the background noise and, since the callers voice is fed back to his receiver, he may hear a mixture of noise and voice in one ear and noise in the other ear. The channel by which noise and speech are fed back to the receiver of a telephone is called the sidetone path. As used herein, the term "sidetone" refers to that portion of an audio spectrum, including the speaker's voice, which is purposely fed back from the microphone to the receiver of a handset to assure the speaker that the line is not "dead" and to help the speaker to find a desirable voice level.
Recent to advances in technology have made this problem more acute One of these technological changes is the mechanical design of the handset. Modern handsets are not as restrictive as earlier handsets in directing sound to the microphone. This loosened restriction allows greater mouth-to-microphone angles and distances for normal conversation but it also renders the telephone more susceptible to interference caused by relatively loud background noise.
Another recent change is the use of an electret microphone instead of a carbon microphone in the handset. The sensitivity of a carbon microphone is non-linear with respect to signal level for low level signals. An electret microphone, on the other hand, exhibits a substantially linear relationship between sensitivity and signal level. Thus, an electret microphone is more likely than a carbon microphone to pick-up interfering background noise.
This problem has been addressed in the past by AT&T in the their LB1026AA, LB1026AB and LB1068AW integrated circuits. Each of these circuits includes a gain expanding preamplifier which attenuates background noise, in pauses between speech, by approximately 10 decibels (dB). These circuits have a relatively complex design, including a preamplifier, a four-quadrant multiplier and an output amplification stage.